1. Technical Field
The present invention relates to a semiconductor device having an insulated gate bipolar transistor (IGBT) and a method of manufacturing the semiconductor device.
2. Related Art
An example of a structure of a semiconductor device having an IGBT in which electrostatic discharge (ESD) resistance is enhanced includes, for example, a structure disclosed in Japanese Unexamined Patent Publication No. H03-211771. This structure is a structure, as shown in FIG. 6, in which a drain diffusion layer of a lateral double-diffused NMOS transistor (LDNMOS) is converted to a reverse conductivity type, an n+ diffusion layer 433 is provided at a position at which it is short-circuited to a p+ diffusion layer 432 functioning as a collector, and then the n+ diffusion layer 433 and the p+ diffusion layer 432 are short-circuited by a collector electrode 434. That is, an n+ diffusion layer 410 serving as an emitter and a p+ diffusion layer 412 serving as a back gate are disposed in parallel to each other in a p− substrate 400, and they are connected to a common emitter electrode. In addition, an n− drift layer 430, a p+ diffusion layer 432 serving as a collector, and the n+ diffusion layer 433 are provided in the p− substrate 400. A collector electrode 434 is connected to the p+ diffusion layer 432 and the n+ diffusion layer 433.
In this structure, since a vertical PNP bipolar transistor is added to the p+ diffusion layer 432, a current flows in the longitudinal direction to some extent, which results in an increase in the ESD resistance.
The following analysis is performed by the present inventor. In the semiconductor device shown in FIG. 6, since the n+ diffusion layer 433 is disposed adjacent to the p+ diffusion layer 432, it is difficult to operate a PNP bipolar transistor included in the IGBT. In detail, in order to operate this bipolar transistor, the product of the resistance R1 of the n− drift layer 430 located under the p+ diffusion layer 432 and the electronic current I1 flowing through the n− drift layer 430 is required to be set to be equal to or more than the built-in potential (for example, about 0.7 V) of a pn diode including the p+ diffusion layer 432 and the n− drift layer 430. However, in the structure shown in FIG. 6, since the n+ diffusion layer 433 is disposed adjacent to the p+ diffusion layer 432, R1 is reduced, and thus it is difficult for the product thereof to exceed the built-in potential. In this case, in the IGBT, the bipolar transistor is difficult to operate. In some cases, only the MOS transistor is operated. For this reason, a saturation current is remarkably reduced.